Over the years, Ethernet has become the most commonly used method for local area networking. The IEEE 802.3 group, the originator of the Ethernet standard, has developed an extension to the standard, known as IEEE 802.3af, that defines supplying power over Ethernet cabling. The IEEE 802.3af standard defines a Power over Ethernet (PoE) system that involves delivering power over unshielded twisted-pair wiring from Power Sourcing Equipment (PSE) to a Powered Device (PD) located at opposite sides of a link. Traditionally, network devices such as IP phones, wireless LAN access points, personal computers and Web cameras, have required two connections: one to a LAN and another to a power supply system. The PoE system eliminates the need for additional outlets and wiring to supply power to network devices. Instead, power is supplied over Ethernet cabling used for data transmission.
As defined in the IEEE 802.3af standard, PSE and PD are non-data entities allowing network devices to supply and draw power using the same generic cabling as is used for data transmission. A PSE is the equipment electrically specified at the point of the physical connection to the cabling, that provides the power to a link. A PSE is typically associated with an Ethernet switch, router, hub or other network switching equipment or midspan device. A PD is a device that is either drawing power or requesting power. PDs may be associated with such devices as digital IP telephones, wireless network access points, PDA or notebook computer docking stations, cell phone chargers and HVAC thermostats.
PSE's main functions are to search the link for a PD requesting power, optionally classify the PD, supply power to the link if a PD is detected, monitor the power on the link, and disconnect power when it is no longer requested or required. A PD participates in the PD detection procedure by presenting PoE detection signature defined by the IEEE 802.3af standard. The PD detection signature has electrical characteristics measured by the PSE, such as a signature resistance in a range from 19 to 26.5 KΩ.
During the PD detection procedure, the PSE produces a detection signal supplied to a device connected to the link. A response signal measured by the PSE indicates a resistance of the device being probed. If the resistance is within the range defined by the IEEE 802.3af standard, the PSE concludes that a valid PD is present.
If the resistance is below the IEEE 802.3af acceptable range, the detection signature may indicate that an improper device is plugged in. In addition, a very low resistance detected during the PD detection procedure may indicate a short circuit condition, or the presence of a Network Interface Card (NIC) connected to the link. A typical NIC impedance is 150Ω. Hence, present PSE's are not able to determine whether a NIC is detected, a short circuit condition exists, or an improper device having a low resistance is plugged in based on a PD response signal measured during the PD detection procedure.
While the presence of a NIC is considered to be a normal condition, it would be desirable to inform a user when an improper device is plugged in or a short circuit condition is detected. Therefore, there is a need for a PD detection scheme that would enable the PSE to distinguish the presence of a NIC from a short circuit condition and the presence of an improper device.